1. Field of the Invention
This invention relates to an electrical arrangement for driving at high speed a rotary tool, e.g. a bit in a surgeon's or dentist's handpiece such as a bur or drill. It is concerned more particularly with a driving arrangement involving an electric motor having a rotor.
2. Prior Art
Known arrangements of this kind comprise an external supply unit and a collector motor mounted in the handpiece, the supply and the motor being connected by a flexible lead having two conductors. The supply unit provides a voltage that can be adjusted, e.g. by means of a pedal, and enables the user of the handpiece to change at will the speed of rotation of the motor and hence of the tool driven by it.
However, the use of a d.c. or a.c. motor having brushes in a dental handpiece has a number of drawbacks.
Firstly, because of the motor's high speed of rotation, at times over 100,000 revolutions per minute, the wear on the brushes is considerable. Secondly, because the motor operates in a moist environment that contains in suspension particles of drilling residue, it is impossible to avoid, despite the sealing means that are provided, deterioration of the brushes and associated parts due to condensation and due to the accumulation of dirt on their surfaces. Thirdly, if the operator uses volatile products, the sparks that are generated by the brushes may cause local explosions.
Brushless d.c. motors that do not suffer from these drawbacks are also known. They are motors having a stator usually with three delta- or star-connected windings, and a magnetized rotor. The windings are energized by an electronic circuit that is controlled by the position of the rotor by means of three stationary sensors on the motor. A motor of this kind for industrial use is for instance described in U.S. Pat. No. 4,338,556.
The presence of three stator windings and of three sensors naturally causes the electrical connections to be more complicated. A motor of this kind requires at least eight connection wires, i.e. three wires for the windings, three wires for the sensors and two wires for the sensors' supply. These eight wires form a flex that is more cumbersome, more rigid and less reliable than a two-wire flex.
This drawback is particularly bothersome in the case of a dentist's drill as the movements of such a drill should only be hampered in the least possible way.